The use of computer systems in the daily operations of business has become widespread. It is not unusual for each worker in a large company to have a personal computer on his or her desk to allow for rapid transfer of messages and information to other workers. These personal computers may be connected to larger computer systems so as to provide access to a richer set of application programs, share expensive printers and disk drives, and offer a more powerful computing environment than otherwise can be offered by a personal computer alone. Workers sharing these systems may be geographically dispersed within a building of many floors or may housed in different buildings in a campus environment.
Yet in order to utilize the capabilities of a central computer system a communications path must be established between each personal computer and the centralized system. A conventional means of coupling each personal computer to a larger shared system is via a coaxial cable connection arranged in a star or radial configuration. Thus each personal computer has an exclusive communication path to the larger system. While a star or radial configuration has the advantage of providing faster communication since the path is not shared by other users, it has the disadvantage that as more personal computers are connected to the computer system, more coaxial cable must be used to create the communication paths. Since the cost of installing the coaxial cable through the ducts and ceilings of a building is a substantial portion of the total cost of providing a personal computer connection to a shared system, a means to reduce the per terminal installation cost is desirable.
Metcalfe et al, U.S. Pat. No. 4,063,220, describes a means of connecting a plurality of computer systems and peripheral devices to a single transmission cable allowing each device on the cable to communicate with all other devices. This approach to connectivity eliminates the need for each communicating computer system or terminal device to be connected to each other by a dedicated transmission path, and thus decreases the complexity of the wiring scheme needed to support the communication.
In order to share access to the common transmission cable among a plurality of communicating devices, the system of Metcalfe requires that all computer systems o terminals transferring information make an initial determination that no other device is presently utilizing the cable. This procedure prevents the collision of data on the cable by separate devices. If it is found that another device is utilizing the medium, transmission can not be initiated until the cable is idle.
Because it is possible for two devices to start a transmission at nearly precisely the same instant, collision can still occur even when both tranmitting devices have adhered to the above rule. Thus in order to avoid this problem, Metcalfe specifies that while transmitting, each communicating device sense the electrical energy level on the cable, and if this energy level is greater than what would be expected due to its own data transmission a collision be declared. Once determining that a collision has occurred, a device must stop transmitting and await another appropriate time to retransmit the information. This technique used to determine whether a collision has occurred on the cable is known as CSMA/CD, Carrier Sense Multiple Access with Collision Detection.
This procedure for transferring information on a single cable while providing for the potential of collision which is described above has matured into an international standard supported by the International Standards Organization and known as the 802.3 standard (ISO 8802/3), which is incorporated by reference into this application. The 802.3 standard specifies a network protocol implementing CSMA/CD. Stations conforming to this standard transmit a specific signal which allow stations to transmit and receive data while detecting collisions with other transmitters. The actual transmitted signal consists of both a DC and AC component. The AC component is used to convey the information or data to be sent, and the DC component is used to implement the collision detection function. Stations attached to the cable can easily detect the carrier by measuring the DC signal level. If more than one station is trying to access the cable at the same time, the DC component will add from each transmitting station. This increased magnitude of DC level can be detected by the attached stations as a collision. The 802.3 standard specifies the transmission media to be a 50 ohm characteristic impedance coaxial cable with a maximum length not to be exceeded. The cable is terminated on either end with matching 50 ohm terminations. When the AC component is transmitted, the signal propogates down the cable in either direction, and terminates at the ends into the 50 ohm terminators. When the DC component is transmitted it is sent out as a constant current. This constant current will develop a voltage on the cable due to the resistance of the cable and the 50 ohm terminators. When the cable is short the resistance of the cable is negligible in comparison with the 50 ohm terminating impedances and the voltage developed is primarily dependent on the terminators. In this case it is relatively easy to determine if a collision occurs on the cable. When the cable becomes longer than the maximum length allowed, the resistance of the cable is no longer negligible. The voltage on the cable is now primarily a function of the cable resistance and DTE placement. Stations on the cable will detect widely different voltage levels from the same current source, and thus DC collision detection becomes difficult if not imposssible to implement. Since the 802.3 standard requires the terminators to be a 50 ohm characteristic impedance, the cable length is defined to be no longer than 500 meters when ultilizing the cable specified by this standard. Longer cable runs can only be accomplished through use of repeaters or amplifiers to boost the energy to an acceptable level for further transmission of the signal along a greater cable length. These repeaters are of course an added expense to network implementation, and thus it is desireable to minimize the need for these devices by increasing the the maximum length of transmissions without using repeaters.
It is a first object of the invention to provide for reliable communication between any two communicating devices in a network while increasing the length of cable that may be used in a network utilizing a collision detection scheme similiar to CSMA/CD.
Another object of the invention is to modify an implementation of the 802.3 standard so as to increase the distances between communicating devices without the addition of repeaters.
A still further object of the invention is to separate the collision detection signal from the transmit or receive signal so as to prevent noise from one signal possibly interferring with the interpretation of the other.